In downhole drilling applications, torque is often transferred from a power source at the surface to a drill bit. In such applications, the torque is conventionally transferred through the shafts and/or cylindrical tubes that make up the drill string. In other downhole drilling applications, torque may be transferred from a downhole drilling motor to the drill bit. In such applications, the torque is transferred through the shafts and/or cylindrical tubes deployed between the drilling motor and the drill bit.
In either of these downhole drilling arrangements, the shafts and/or cylindrical tubes deployed between the power source and the drill bit must also typically accommodate significant bending loads, such as are commonly encountered in deviated boreholes. Such torsional and bending loads must typically be transferred through joints (e.g., threaded box and pin connections) at which adjacent components (e.g., drill string tubes) are connected.
In certain downhole applications, torque is transferred from a cylindrical tube (such as the tubes that make up the drill string) to a shaft (such as a rotatable shaft in a downhole tool). One common way to accommodate torsional and bending loads in such applications is to thread a mating sleeve to the shaft and tighten it (with sufficient torque to withstand anticipated torsional and bending loads) against a shoulder on the outer diameter of the shaft. Alternatively, another common approach includes using a second threaded sleeve as a shoulder. The mating sleeve is then tightened against the second sleeve (e.g., similar to a conventional locking nut). While such methods have been utilized to connect shafts and sleeves in various drilling operations, one drawback is that they typically require the sleeves to be tightened with a make up torque equal to about one-half the torsion yield of the connector (e.g., to about 45,000 foot pounds in common downhole applications). The use of such a high make up torque increases the stresses in the shaft and sleeves and thus tends to limit the bending and torsion loads that they may safely withstand.
Locking devices for downhole box and pin connectors are also known in the prior art. FIG. 1 shows one such prior art locking device that includes three cylindrical sleeves 75, 90, and 95 deployed coaxially about the box 80 and pin 70. Torque sleeve 75 includes a plurality of dogs 73 formed on an inner surface thereof and is configured to engage corresponding slots 72 formed on an outer surface of the pin member 70. Slide sleeve 95 includes a plurality of dogs 83 formed on an inner surface thereof is configured to engage slots 82 formed on an outer surface of the box member 80. Alignment sleeve 90 includes first and second sets of radial splines 79 and 91 formed on an internal surface thereof. The first set 79 includes 41 splines and is configured to engage splines 78 formed on an outer surface of torque ring 75. The second set 91 includes 40 splines and is configured to engage splines 92 formed on an outer surface of the slide ring 95. As such, the pin member 70 is rotationally coupled to the box member 80 when spines 78 and 79 and splines 91 and 92 are engaged.
With continue reference to FIG. 1, the locking device is made up about the connection by deploying the torque sleeve 75 about pin member 70 with dogs 73 engaged with slots 72. Alignment sleeve 90 and slide sleeve 95 are deployed about box member 80 with dogs 83 engaged with slots 82. The pin member 70 is then threaded to box member 80 (with threads 71 engaging threads 81) and tightened to a predetermined torque. A spacer (not shown) is translated into place and holds torque sleeve 75 firmly in place against shoulder 85 of box member 80. The alignment sleeve 90 is then rotated between the torque ring 75 and slide sleeve 95 until splines 78 and 79 and splines 91 and 92 are aligned. The slide sleeve is then translated axially towards the torque ring such that splines 78 and 79 and splines 91 and 92 engage. One or more soft nails are then driven into grooves 98 to hold sleeves 75, 90, and 95 together.
Another prior art locking device for a downhole box and pin connector includes two sleeves deployed coaxially about the threaded box member. The first sleeve includes four large axial dogs on one axial face disposed to engage four corresponding radial slots disposed on the outer surface of the box member. The second sleeve includes a plurality of axial splines configured to engage with corresponding axial splines disposed on a shoulder portion of the threaded pin member. The sleeves are further disposed to engage one another via a plurality of interlocking axial splines. As such, the box member may be rotationally coupled to the pin member when the first and second sleeves are engaged. The second sleeve includes 19 axial splines on one axial face and 20 axial splines on the opposing axial face. The locking device further includes a covering member deployed about the first and second sleeves. The covering member is intended to provide axial support for the first and second sleeves and is held in place by a soft nail pressed into a circumferential groove in the shoulder portion of the pin member.
The above-described connector assemblies, while potentially serviceable, are complex (e.g., including three splined sleeves nailed together and held in place by a spacer or including two splined sleeves with an encircling cover nailed in place). Moreover, the above described connector assemblies do not efficiently utilize the available diametrical space. For example, in both assemblies the sleeves are deployed about the box and pin members, thereby reducing the maximum outer diameter of the box and pin members. Such a reduced outer diameter tends to reduce the strength of the connection. Therefore, there exists a need for improved connector assemblies for downhole drilling tools. In particular, there exists a need for connector assemblies for connecting a mating sleeve to a rotatable shaft that are suitable to support large torsional and/or bending loads, often cyclic in nature, and that efficiently utilize the diametrical space available in the downhole tool.